Radiotherapy is a method of treating tumours and like disorders by directing a beam of ionising radiation towards the tumour site. This radiation causes damage to tissues that it passes through, which inhibits and gradually reduces the tumour. It also causes damage to healthy tissue, albeit at a slightly lesser rate, so the beam is usually collimated to a cross-sectional shape that reflects the tumour shape; this may be the projected shape of the tumour along the current beam axis, or it may be some other shape designed to build up a specified dose distribution in the patient. In addition, the beam is also directed towards the patient from a number of different angles.
These measures for limiting the dose applied to healthy tissue and ensuring the adequate dose is applied to the tumour do depend on the patient being in a predictable location, and remaining there. Generally, prior to each treatment fraction a patient is positioned carefully by a clinician so as to place them in the correct position and orientation relative to the radiotherapy apparatus. The patient will also usually be supported on an adjustable couch that will allow fine-tuning of their position, preferably in all six degrees of freedom (three translational and three rotational), but sometimes fewer degrees of freedom such as the three translational degrees only, depending on the nature of the radiotherapy apparatus. Most radiotherapy apparatus also includes a means for monitoring the current patient position in real time, such as an IR based tracking system, a video camera, or the like.
Arrangements also exist for immobilising a patient; whilst these are often effective, they can be quite invasive and it may be desirable to avoid them where possible, depending on the circumstances of the patient.
Currently, if the monitoring systems detect that the patient has moved too much, the treatment is paused or aborted. This is undesirable in that it causes delay, but is better than continuing a treatment where the patient is misaligned.